1. Field of the Invention
The present invention relates to a telephone subscriber line interface circuit.
2. Description of the Prior Art
In digital telephone switching systems the subscriber line interface circuit is required to meet the various functions known as BORSHT which is an acronym of Battery feed, Over-voltage protection, Ringing current supply, Supervisory of line status, Hybrid circuit (two-wire to four-wire conversion) and Test of subscriber lines. Although many attempts have hitherto been made to implement these functions in integrated circuits for purposes of achieving compactness, improving reliability and reducing power dissipation, the proposed subscriber line interface circuits are not satisfactory in a number of aspects.
U.S. Pat. No. 4,387,273 granted to Ramon C. W. Chea, Jr. discloses a subscriber line interface circuit with an impedance synthesizer. The impedance synthesizer includes two resistors having their first ends coupled respectively to the subscriber line terminals and two voltage amplifiers which are connected to the opposite ends of the resistors, respectively. The voltage amplifiers are driven by a differential line voltage between the line terminals which is supplied through a circuit having a predetermined transfer function. In the disclosed synthesizer, the transfer function is set equal to a constant value k which is implemented by an inverting amplifier. If the gain of each voltage amplifier is unity, the impedance Z between the subscriber line terminals is represented by: EQU Z=2R/(1-2K) (1)
where, R is the values of the aforesaid resistors.
The transfer function K must satisfy following equation to obtain the impedance Z. EQU K=(1/2)-(R/Z) (2)
Since Equation 2 must be computed to determine the transfer function K, it is impossible to establish a one-to-one correspondence between the desired impedance and transfer function. However, from the circuit design standpoint it is required that the transfer function be an integral multiple of the desired impedance.
A Transformerless hybrid circuit for circuit integration is described in Articles "Design and Performance of Subscriber line Interface Module for Digital Switching System", ISSCC 1980, by Keiichi Yasuda et al. and "A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface", IEEE Journal of Solid-State Circuits, Vol. SC-16, No. 4, August 1981, Denis W. Aull et al. The hybrid circuits discussed in these publications include a crosstalk canceller which is designed so that it has no adverse effect on the transmission characteristic of the path from the input terminal of the four-wire circuit to the subscriber line and requires at least three impedance networks for synthesizing a terminal impedance Z.sub.T, a transmission impedance Z.sub.S and a balancing impedance Z.sub.B is used to balance against the load impedance Z.sub.L. However, even if the impedance Z.sub.B is balanced against the load impedance, distortion can occur in the transmission characteristic of the path from the input line of four-wire circuit to the two-wire subscriber line if the terminal impedance Z.sub.T is not equal to the load impedance Z.sub.L. This arises from the fact that, since the transmission path is represented by a source having an impedance Z.sub.S =Z.sub.T and a load impedance Z.sub.L coupled thereto, the ratio Z.sub.L /Z.sub.S becomes a non-real number and hence the transmission path acquires a particular response characteristic in the frequency spectrum.
It is further desired that battery feed and dial pulse detection be provided by a single circuit and a variety of proposals have hitherto been made. Most these proposals include an active circuit using capacitors to synthesize an inductance equivalent to the conventional feeding coil. However, these proposals are still unsatisfactory for dial pulse detection because of the inherent difficulty to provide a fast response characteristic.